Let's assume the SiteA RoadRunner tunnel is 10.42.42.0/24
On SiteB site-to-site Remote Networks put 10.0.0.0/16,10.42.42.0/24
On Site A RoadRunner server Local Network/s put 10.0.0.0/16,192.168.0.0/16
Then routing will work.

Make sure rules on OpenVPN at SiteA and SiteB allow the traffic to/from those subnets.
Then firewalling will allow the traffic.

I connect in like this all the time, to 1 office, and use the whole internal network across lots of offices.